Add a supported extension API for collection list-view tabs

[cbratschi]

Describe the Feature

Plugins can add custom collection views, but there is no supported way to append navigation tabs beside Payload’s built-in All, Hierarchy/Folders, and Trash tabs.

DefaultListView is public, but it hardcodes CollectionListHeader. The internal DefaultListViewTabs component is not exported, and the available beforeList, afterList, table, menu-item, and view-action slots do not render inside the list-header tab group.

Current Workaround

A plugin that adds a custom collection subview must either replace and reproduce the list header or render DefaultListView and inject a tab afterward using a DOM portal and private selectors such as .list-header__actions and .default-list-view-tabs.

Custom subviews must separately duplicate Payload’s built-in tab URLs, labels, active states, folder/hierarchy handling, trash handling, and styles.

Proposed Direction

Add a supported list-view tab composition point. This could be a collection configuration slot such as admin.components.views.list.tabs, or a public composable DefaultListViewTabs API together with a way for DefaultListView / CollectionListHeader to append custom tabs.

Useful tab context would include the collection slug/config, current view type, and whether the view is rendered inside a drawer. Custom tabs should be excluded from drawers by default.

I would be happy to implement the preferred API and add focused tests once the desired shape is agreed.